Both nutrients are crucial for healthy bones and specialists advise getting as much as possible from a good diet. The body also makes vitamin D from sunshine. If an older person has a vitamin deficiency or bone-thinning osteoporosis, doctors often prescribe higher-than-normal doses.

But for otherwise healthy postmenopausal women, adding modest supplements to their diet - about 400 international units of D and 1,000 milligrams of calcium - don’t prevent broken bones but can increase the risk of kidney stones, the U.S. Preventive Services Task Force said Monday.

It isn’t clear if those doses offer bone protection if taken before menopause, or if they help men’s bones, the guidelines said.

What about higher-dose supplements that have become more common recently? There’s not enough evidence to tell if they would prevent fractures, either, in an otherwise healthy person, the panel concluded. It urged more research to settle the issue.

It’s a confusing message considering that for years, calcium and vitamin D supplements have been widely considered an insurance policy against osteoporosis, with little down side to taking them.

"Regrettably, we don’t have as much information as we would like to have about a substance that has been around a long time and we used to think we understood," said Dr. Virginia Moyer of the Baylor College of Medicine, who heads the task force. "Turns out, there’s a lot more to learn."

The main caution: These recommendations aren’t for people at high risk of weak bones, including older adults who have previously broken a bone and are at risk for doing so again, said Dr. Sundeep Khosla of the American Society for Bone and Mineral Research. Those people should consult a doctor, said Khosla, a bone specialist at the Mayo Clinic who wasn’t part of the panel’s deliberations.

Calcium and vitamin D work together, and you need a lifetime of both to build and maintain strong bones. Vitamin D also is being studied for possibly preventing cancer and certain other diseases, something that Monday’s guidelines don’t address and that other health groups have cautioned isn’t yet proven.

For now, national standards advise the average adult to get about 1,000 mg of calcium, 1,300 for postmenopausal women, every day. For vitamin D, the goal is 600 IUs of vitamin D every day, moving to 800 after age 70, according to the Institute of Medicine, which set those levels in 2010. The nutrients can come from various foods, including orange juice fortified with calcium and D; dairy foods such as milk, yogurt and cheese; certain fish including salmon; and fortified breakfast cereals. Harder to measure is how much vitamin D the body also produces from sunshine.

Most people should get enough calcium from food, said Mayo’s Khosla. But while he cautions against too high doses, he frequently tells his patients to take a multivitamin because it’s harder to get vitamin D from food and during the winter.

While supplement science gets sorted out, the task force’s Moyer advises healthy seniors to exercise - proven to shore up bones and good for the rest of the body, too.

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